The present invention relates generally to fluid-powered systems, and, more particularly, to an improved sensor for determining the level of particulate contaminants in a system fluid.
Contaminated fluids can cause erratic operation or catastrophic failure of pumps, valves and actuators used in hydraulic control systems. Many aircraft fuel systems use servo-based technology, and contamination can lead to dangerous failures. This is often exacerbated by the fact that these are typically xe2x80x9ctotal lossxe2x80x9d failures.
Fluid cleanliness is difficult to monitor. Lubricating oil systems for high-value plant and machinery, such as gas and steam turbines, must be maintained to an exceptionally high standard if major damage and loss of production is to be avoided. Traditional ways of monitoring oil cleanliness are inconvenient and expensive, and often the results are only available after considerable delay.
The contamination sensitivity of various elements of a fluid power system is fairly well established. For pumps and motors, there is a critical range of particulate size which will cause most damage. However, a servovalve can experience erratic operation and accelerated wear due to particles as small as a few microns impacting on, and accumulating around, the lands of a valve spool. These potential failure modes are addressed and guarded against by specific design features that are built into the servovalve. The same is true of the other components within the system. However, experience has shown that there is still a strong correlation between the cleanliness of the fluid and the reliability of the system in which it is used.
Advances in fluids, filtration and component design have resulted in improvements. As a result, stable hydraulic systems are generally reliable, but problems arise when parameters change. For example, a pump might fail, sending a cloud of debris through the system. The fluid may overheat and create a multitude of tiny hard particles, New lubricating oil, added to the tank might not be quite as clean as was thought. Fuel oil (e.g., aircraft fuel) does not re-circulate through the system, so there is not the opportunity for it to sequentially pass through various filters. Such occurrences can wreak havoc with even a well-designed system, and, being unexpected and random in nature, can jeopardize both plant and safety in an unpredictable manner.
Traditionally, cleanliness has been monitored by taking a sample of the fluid, and then assessing the number and size of contaminant particles in such sample. This can be done either manually (e.g., by using a microscope) or automatically (e.g., usually employing some sort of light-blockage technique). Both techniques are essentially laboratory procedures. Although it is true that most of these instruments are portable, and, in some cases, can even be connected directly to the fluid system to be tested, none is regarded as being small or robust enough to be permanently installed.
Accordingly, there is believed to be a need for a contaminant sensor which is small, robust and suitable for permanent installation in high-value and/or safety-critical equipment. A system potentially offering these features has been developed by others (see, e.g., published British Pat. Application No. 2,361,548), but o g to the necessary size of the flow-measuring piston required, it is still awkwardly large. The inventive device presented herein provides a novel technique which permits the piston size to be dramatically reduced.
With parenthetical reference to the corresponding parts, portions or surfaces of the disclosed embodiment, merely for purposes of illustration and not by way of limitation, the present invention provides an improvement in apparatus (20) for testing the level of contaminants in a fluid during a test period.
The apparatus has a source of pressurized fluid (Ps) to be tested; a sump (R); a test passage (48) which is substantially free of occluding contaminants at the beginning of the test period and which is adapted to be supplied with a flow of fluid from the source, the passage being so configured and arranged as to be progressively occluded by contaminants in the fluid flow; and a device defining a variable-volume chamber (47), the volume of the chamber being variable between a minimum value and a maximum value. The improvement broadly comprises a valve (36) operatively arranged to direct fluid that has passed trough the test passage (48) to the chamber (47) when it is desired to expand the volume of the chamber, and arranged to direct fluid that has passed through the test passage to the sump, and to permit fluid in the chamber to flow to the sump, when it is desired to contract the volume of the chamber, a return mechanism (24) for selectively urging the chamber to contract, a calibrated sensor (44) for measuring the volume of the chamber; a controller (50) for operating the flush and recycle valves to selectively cause the chamber to repeatedly expand and contract cyclically during the test period; and a timer (53) for measuring the time required for the fluid flow through the test passage to cause the chamber to increase from a first volume to a second volume; whereby the flow through the test passage during each of the successive cycles of chamber volume may be calculated.
The level of contamination in the fluid may be determined as a function of the flow through the test passage during the first cycle and the number of cycles required for such flow to be reduced to a predetermined minimum value. The test passage may have an annular transverse crossection, such as defined between the facing surfaces on a valve spool land and a cylindrical bushing (not shown) mounted on the body. The device may have a piston (39) mounted for sealed sliding movement within a cylinder (40). The valve (36) may be a relief valve. The return mechanism may be fluid powered, and the controller may cause the chamber to expand or contract cyclically as a function of the flow through the passage.
Accordingly, the general object of the invention is to provide a fluid contaminant sensor. Another object is to provide an improved fluid contaminant sensor which is adapted to be mounted on, or otherwise permanently associated with, the system in which the fluid is contained.
Still another object is to provide a fluid contaminant sensor which closely simulates the passages that are likely to be encountered in the accompanying fluid system, and which is able to simulate and predict the effect of such contaminants on flow through the system.
These and other objects and advantages will become apparent from the foregoing and ongoing written specification, the drawings, and the appended claims.